Dehydrocyclization is a well known reaction wherein alkanes are converted to aromatics. For example, hexane may be dehydrocyclized to benzene, heptane to toluene, etc.
Catalytic reforming is a well-known refinery process for upgrading light hydrocarbon feedstocks, frequently referred to as naphtha feedstocks. Products from catalytic reforming can include high octane gasoline, useful as automobile fuel, and/or aromatics, such as benzene and toluene, useful as chemicals. Reactions typically involved in catalytic reforming include dehydrocyclization, isomerization and dehydrogenation. The dehydrocyclization and dehydrogenation reactions lead to the production of aromatics starting, respectively, from linear and slightly branched alkanes of a proper size, e.g., 6-8 carbon atoms and from cycloalkanes, e.g., cyclohexane and methyl-, dimethyl- and ethyl- cyclohexanes. Because typical petroleum feedstocks are much richer in linear and slightly branched alkane than in cycloalkanes, dehydrocyclization tends to be the more important of these reactions. Thus, reforming typically includes dehydrocyclization. However, dehydrocyclization or aromatization of alkanes can be directed more narrowly than reforming.
Reforming is often carried out by passing an initial naphtha through a plurality of reactors wherein each reactor is usually a single reforming stage wherein the RON and the aromatics content of the reformate from each succeeding reforming reactor is higher than from the last preceding reactor until that of the final reactor is a desired value, for example 100 RON or greater with a corresponding increase in aromatic content. In multistage reforming processes the same catalyst may be used in each of the reforming stages or different catalysts can be used in different stages.
Generally, because of the overall endothermicity of the reforming reactions prior art multistage processes have used interstage heating to provide roughly equal inlet temperatures. Because of the relatively large endothermicity of some of the easier to catalyze reforming reactions the first stage has generally been the smallest stage so that the temperature drop occurring in that stage has been minimized. Furthermore, the octane and the aromatic content of the reformate of all stages preceding the final stage has generally not been controlled or even monitored.
To improve C.sub.5+ liquid volume yield of reformate of a desired RON it is known to utilize, for example, a first catalyst in a preliminary reforming stage or stages to produce a partially reformed reformate with the preliminary stage or stages operating at a relatively higher pressure and then to utilize a second catalyst in the final reforming stage with the pressure in the final reforming stage being different (generally lower) than that in the preliminary reforming stage or stages. In this manner the catalysts in each of the stages are utilized under conditions which lead to a maximum C.sub.5+ liquid volume yield for that particular catalyst consistent with its stability characteristics. For example, the catalyst or catalysts in the preliminary stage or stages might be particularly useful for promoting such reactions as isomerization and dehydrogenation while the catalyst used in the final stage might be particularly advantageous for carrying out dehydrocyclization reactions while minimizing hydrocracking reactions.
Representative of prior art patents in the area of multistage reforming is U.S. Pat. No. 4,627,909 of R. C. Robinson, issued Dec. 9, 1986. The process of this patent involves two-stage reforming with the second stage being at lower pressure than the first stage. Because the catalyst life is short at lower pressures swing reactors are used in the second stage. A large pore size zeolite is the preferred catalyst for the second reforming zone. This process is not designed to provide optimum AB/CD production of product reformate having a selected RON.
Another process of the prior art is described in U.S. Pat. No. 4,443,326 of L. A. Field, issued Apr. 17, 1984. This patent likewise teaches a two stage process but in the case of this patent the second stage catalyst does not utilize a metal component to promote dehydrocyclization. Instead, paraffins in the first stage reformate are cracked to olefins which are recombined at relatively high temperatures to form aromatics. No attempt is apparently made to optimize AB/CD production of reformate.
U.S. Pat. No. 3,899,411, issued to A. C. Bonacci and W. P. Burgess on Aug. 12, 1975 shows still another prior art multistage reforming process. This process utilizes a small pore size shape selective conversion catalyst in the second stage. The second stage is, however, basically a cracking stage as opposed to a reforming stage. Again, there is no teaching of optimizing AB/CD production in the product reformate.
U.S. Pat. No. 4,808,295, issued Feb. 28, 1989 to M. Nemet Mavrodin relates to a two-stage process for converting a predominantly C.sub.2 -C.sub.10 aliphatic feed to benzene. This patent does not suggest optimizing AB/CD production of product.
At times it is desirable in the operation of refineries to provide a maximum production of high octane gasoline on a day to day basis. A term which is utilized to describe such an output is that of octane barrels per calendar day (OB/CD) above a so-called "pool octane" (often in the 86-96 RON range, e.g., 93 RON)which is the average octane of the gasoline produced in a refinery of interest. If the OB/CD for a given set of reformers in a refinery, can be increased this leads to a direct increase in the octane and/or volume of the final product which can be marketed. This also leads to an increase in aromatics content of the final product. Thus, it would be desirable to provide a method for optimizing OB/CD production from a multistage reforming process whereby AB/CD can likewise be optimized.